SERS chemical enhancement by copper - Nanostructures: Theoretical study of Thiram pesticide adsorbed on Cu₂₀ cluster

Download

Cite this paper: Vietnam J. Chem., 2021, 59(2), 159-166

DOI: 10.1002/vjch.202000137

Article

SERS chemical enhancement by copper - nanostructures: Theoretical

study of Thiram pesticide adsorbed on Cu₂₀cluster

Truong Dinh Hieu^1,2, Ngo Thi Chinh^1,2, Nguyen Thi Ai Nhung³, Duong Tuan Quang⁴,

Dao Duy Quang^1,5*

¹Institute of Research and Development, Duy Tan University, Da Nang, 50000, Viet Nam

²Faculty of Natural Sciences, Duy Tan University, Da Nang, 50000, Viet Nam

³Department of Chemistry, University of Sciences, Hue University, Hue City, Thua Thien Hue 49000, Viet Nam

⁴University of Education, Hue University, Hue City, Thua Thien Hue 49000, Viet Nam

⁵Department of Environmental and Chemical Engineering, Duy Tan University, Da Nang, 50000, Viet Nam

Submitted August 7, 2020; Accepted November 9, 2020

Abstract

Surface-enhanced Raman spectroscopy (SERS), a surface-sensitive technique, allows the practicability of detecting

chemical compounds in ultra-low concentration. In this work, a chemical enhancement mechanism of SER process of

Thiram pesticide adsorbed on copper nanomaterial surface was proposed based on density functional theory (DFT)

approaches. Structural and electronic properties of Thiram and Thiram-Cu₂₀complexes were optimized using PBE

method with LanL2DZ basis set for copper atoms and cc-pVDZ basis set for the non-metal atoms. In the most stable

adsorption configuration, Thiram interacted with Cu₂₀cluster via two S(sp²) atoms. The main peaks on normal Raman

spectrum of Thiram were characterized at 371, 576, 1414 and 1456 cm^-1responsible for the stretching vibrations of

C–S, C=S, S–C–S and C–N groups, respectively. Otherwise, the main peaks of Thiram-Cu₂₀SERS spectrum were

found at 534, 874, 982, 1398 and 1526 cm^-1corresponding to the stretching vibrations of S–S, C-S, S–C–S, C–N and

CH₃–N bonds, respectively. The SERS chemical enhancement of Thiram by Cu₂₀cluster was about 2 and 6 times

stronger than those obtained from Ag₂₀and Au₂₀cluster, respectively. The chemical enhancement mechanism was also

explained by analyzing HOMO and LUMO energies gap and density of states.

Keywords. Thiram, copper cluster, Raman, SERS, DFT.

1. INTRODUCTION

This compound is also used to protect fruit trees and

ornamental fruits from damage of rabbit, rodent and

Pesticides are chemical compounds used in modern deer.^[7]

agriculture to kill insects, fungus, bacteria, weed and

For many decades, surface-enhanced Raman

rodents. They are respectively named as insecticides, spectroscopy (SERS) has intensively been

fungicides, bactericides, herbicides and rodenticides. investigated

for

its

electromagnetic

field

By the structure, pesticides can be divided into enhancement near the nano-scale metallic surfaces

organochlorines, organophosphates, carbamates and of coinage metals (i.e. gold, silver and copper).

triazines.^[1,2]An increasing utilization of pesticides Despite of intensive research attempts SERS

in agriculture results in several severe problems on chemical enhancement mechanism is still unclear

environment and human health.

mainly due to the relatively complicated enhancing

Thiram (tetramethyl-thiuram disulfide or factors and inconsistent experimental results. The

bis(dimethyl-thiocarbamoyl) disulfide) (C₆H₁₂N₂S₄) advantages of SERS are that it magnifies Raman

is a carbamate-categorized pesticide. Its molecular signals corresponding to the adsorbed compounds

structure has two dimethyl-dithio-carbamate groups from 10⁶to 10¹⁰times. Therefore, SERS technique

– (CH₃)₂N–CS₂linked together by a disulfide bridge has increasingly been utilized to improve detection

(S–S). Thiram has been used in many countries as of chemicals at trace concentrations. Attracted by its

fungicide to protect fruits, vegetables, ornamental great advantages, many researches have employed

and turf crops from a variety of fungal diseases.^[3-6]SERS to analyze different chemical pesticides,

Vietnam Journal of Chemistry

Dao Duy Quang et al.

including Thiram, accumulated either in the predicted and compared with the spectra obtained by

environment or in agricultural products. the corresponding investigations on Ag₂₀and Au₂₀

Kang et al. analyzed the SERS spectrum of clusters in order to demonstrate the effectiveness of

Thiram adsorbed on silver surface.^[8]Their results copper substrate for SERS technique. Finally, a

revealed that the peaks of Thiram located in the chemical enhancement mechanism is proposed

region below 1000 cm^-1(related to C–S, C–S–S providing more insight for SERS phenomenon. To

assignments) are decreased or even disappeared in the best of our knowledge, there have been an

the SERS spectrum; whereas, others characterized insignificant number of experimental and

for C–N and CH₃NC are enhanced, especially C–N computational studies in the literature on SERS of

stretching mode at 1372 cm^-1. These phenomena chemical compounds adsorbed onto the surface of

were also confirmed by Verma et al. using silver copper nanoparticles.

nanodendrites.^[9]

The prediction of Raman and SERS spectra has 2. COMPUTATIONAL METHOD

commonly been investigated using density

functional theory (DFT). Metallic cluster models are All the DFT calculations were carried out using

often used to reproduce nanoparticle surface. The Gaussian 16, revision A.03.^[12]The chosen DFT

complexes produced from interaction between an method was PBE^[13]combined with the LanL2DZ

analyzed ligand and a metallic cluster can be utilized basis set for metallic atom (i.e. Cu, Au, Ag) and the

to predict their SERS spectra. Rajalakshmi et al. cc-pVDZ basis set for the non-metallic atoms.

determined geometrical and electronic structures of Different configurations of interactions between

2-propylpiridine-4-carbothioamide as well as studied Thiram and Cu clusters were analyzed. The most

infrared, Raman spectra.^[10]In their work, various stable Thiram-Cu₂₀complex was used to project the

DFT functionals including PBEPBE, SVWN, corresponding SERS spectrum. It was then

HCTH, B3LYP, mPW1PW91, B3PW91 combined compared with the spectra obtained from Thiram-

with aug-cc-pVDZ basis set were chosen as Au₂₀and Thiram-Ag₂₀complexes in an attempt to

computational strategies for spectra prediction. The explain the influence of cluster nature on SERS

research indicated that the B3LYP/aug-cc-pVDZ enhancement. The scaling factor for harmonic

model results in the lowest deviations in the frequencies of PBE/cc-pVDZ method was 1.0353.^[14]

prediction of structure and vibrational spectra. Gaussum^[15]was used to investigate density of states.

Recently, An et al. investigated surface-enhanced

Raman scattering of melamine (C₃H₆N₆) on silver 3. RESULTS AND DISCUSSION

substrate using experimental and DFT studies with

the B3LYP/6-31G(d) method.^[11]Silver cluster

3.1. Structure of Thiram

models including Ag₄, Ag₈, Ag₁₀and Ag₂₀were used

to reproduce silver substrate. It was found that the Figure 2 shows optimized structure, the highest

small size clusters like Ag₄can be an effective occupied molecular orbital (HOMO), the lowest

predictor for Raman and SERS spectra of melamine. unoccupied molecular orbital (LUMO) distributions

This research also showed that the enhancement of and electrostatic potential (ESP) map of Thiram in

typical peaks localized at 676 and 983 cm^-1were vacuum. The experimental structural parameters of

correctly predicted and consistent with the Thiram are also included.

corresponding experimental results.

The structural parameters obtained from the

PBE/cc-pVDZ level of theory are in good agreement

with the experimental values. The difference

between the respective bond lengths varies from

0.020 to 0.042 Å, within the deviation of 1.4-2.8 %.

The calculated C11–S1–S2–C12 dihedral angle is

86.6^owhich is in accordance with the measurement

gained from experiments (i.e. 88.3^o). In the C₂NCS₂

group, all the atoms are nearly coplanar given the

data S3–C11–N5–C7 = 4.7^o, S1–C11–N5–C8 =

3.5^o, S1–S3–C7–C8 = 4.2^o. These imply a sp²-

hybridized structure of N and C atoms. In fact, C7–

N5–C8 and C7–N5–C11 angles are equal to 118.6^o

and 118.0^o, respectively, which are far from the

characteristic angle of an sp³hybridization (109.5^o);

Figure 1: Molecular structure of Thiram

This study investigates Raman and SERS spectra

of Thiram pesticide (figure 1) adsorbed on copper

substrate using Cu₂₀cluster model. The Raman

spectrum of Thiram is projected and compared with

the experimental data from the literature. SERS

spectrum of Thiram adsorbed on Cu₂₀cluster is

Vietnam Journal of Chemistry

SERS chemical enhancement by copper - nanostructures:…

also S3–C11–N5 (124.9^o) and S1–C11–S3 (124.0^o) and the S(sp²) atoms of Thiram are predicted

angles are also close to 120^o, the typical angle of an exhibiting high tendency to donate electrons.

sp²hybridization. The bond angles at each S atom of Otherwise, LUMOs are mainly distributed around S,

S–S bridge (i.e. C12–S2–S1 and C11–S1–S2) are N and C(sp³) atoms. Thus, if a Thiram molecule is

102.5^o. These values are smaller than sp³angle allowed to interact with a metal cluster, these

(109.47^o). The reason of this is the influence of two regions are expected to accept electrons and form an

free electron pairs in each S atom. These electron interactive bond. The results are also highly

pairs occupy large space and make the C–S–S bond consistent with those gained from ESP map.

angle smaller. But this difference is not too big.

If Thiram is adsorbed onto the surface of a

copper crystal, electron transfer may occur. In detail,

the regions in the adsorbent molecule localized by

high negative potential or large HOMOs may

interact with the copper cluster via donation of

electrons to the clustering atoms. Reversely, the

regions owing high positive charge or large LUMOs

are predicted to accept electrons transferred from

copper cluster. The stronger electronic exchange

formed, the more stable the complexes. In particular,

Thiram may interact with copper cluster via the

position of atoms S, especially at the S(sp²) atoms.

Therefore, S1 and S2 are in sp³hybridization.

3.2. Structure of Thiram-Cu₂₀complexes

Figure 3 shows structures and relative energies of

seven complexes representing for all possible

interactions between a Thiram molecule and Cu₂₀

cluster. Relative energy of each complex is

calculated by the difference of the according

enthalpy value with the lowest one.

Figure 2: (A) Optimized structure, (B) ESP map,

lengths are in Å, angles are in degree. Values in

parentheses highlighted in red color are

experimental values of Kang et al.^[8]

Thiram molecule attends to interact with at a

edge of pyramidal Cu₂₀cluster via two or three

sulfur atoms. The interacting modes A and B are

obtained by the complexation between two S(sp²)

atoms (i.e. S3 and S4) with two copper atoms. In

detail, the complex A comprises the interactions

occurring at top of the cluster, and the interaction in

complex B is observed at the center of one edge on

the pyramidal cluster. Mode A is the most stable

complex with the lowest relative energy (∆E = 0.0

kcal/mol). This is followed by mode B with the

value of ∆E 4.2 kcal/mol higher. Modes C (∆E = 7.4

kcal/mol) and D (∆E = 12.9 kcal/mol) correspond to

the interactions at S2 and S3 atoms with two other

respective copper atoms located at the top and at the

edge of the copper clusters, respectively. Finally, the

adsorption modes E, F and G are built through the

interactions between 3 sulfur atoms (one S(sp²) atom

and two S(sp³) atoms) with the copper cluster. The

relative energies of these modes are significantly

higher than the energy of mode A, varying within

9.6-12.6 kcal/mol. Thus, a Thiram-Cu₂₀complex is

predicted most stable if the S(sp²) atoms in the

Thiram molecule interact with cluster-copper atom

at the top of the cluster.

The ESP map given by figure 2B illustrates the

charge distribution of molecules in a three-

dimentional

simulation,

which

allows

determination on how the molecule interacts with

exotic agents. In principle, the regions colorized in

red represent the most negative atomic zones, prone

to be attacked by electrophilic species; whilst, blue

regions exhibit the most positive charges, conducive

to an interaction with nucleophile species. This

suggests that the S(sp²) atoms possess the highest

negative potential due to the +M effect of

neighboring N and S(sp³) atoms, while the most

positive potential is observable localizing at SS and

CH₃groups. The high significance of positive charge

at these groups can be explained by the –M effect of

the C=S bond and the electron deficiency on N

atom. Therefore, S(sp²) atoms are expected to donate

electrons to an external electrophilic agent; whereas,

S–S and CH₃groups can are more likely to accept

electrons from a nucleophilic counterpart.

HOMO and LUMO distributions in Thiram

structure are shown in Figures 2C and 2D. HOMOs

localize around the C–S–S–C group and two S(sp²)

atoms. Besides, N and C(sp³) atoms are surrounded

by smaller HOMOs. Therefore, the C–S–S–C group

Vietnam Journal of Chemistry

Dao Duy Quang et al.

In addition, the S(sp²) atoms are more favorable molecule and Cu₂₀cluster may perform quasi-

to approach the cluster than their sp³– hybridization covalent characteristics, conducive to the stability of

counterparts. The interactive distances between the bonding, especially formed by complex A.

S(sp²) atom (i.e. S3 and S4) and copper atoms vary

In the next section, the SERS spectrum of the

between 2.33-2.50 Å while the corresponding complex A in comparison with the normal Raman

figures for S(sp²) atoms (i.e. S1 and S2) are in the spectrum of Thiram are projected in order to propose

range 2.44-2.75 Å. These values the covalent bond a chemical enhancement mechanism of Thiram

distance of Cu-S (2.37 Å).^[16]Therefore, in these adsorbed on the a Cu₂₀cluster.

complexes, the interactions between a Thiram

Figure 3: Optimized structures and relative energies (E, in kcal/mol) of seven Thiram-Cu₂₀complexes

A-G. Unit of distance is Å

differences with the normal Raman spectrum should

be noted. In figure 4B, the most marked peak is

3.3. Normal Raman and SERS spectra

Figure 4 compared Raman spectrum of Thiram found at 982 cm^-1and the other significant peaks are

(figure 4A) and SERS of the most stable Thiram- also detected at 534, 1398 and 1526 cm^-1. They are

Cu₂₀(figure 4B), Thiram-Ag₂₀(figure 4C) and Au₂₀- assigned to the stretching vibrations of S–S, S–C–S,

Thiram complexes (figure 4D). In addition, the C–N, CH₃–N bonds; the scissoring bending

Raman and SERS vibrational assignments are listed vibrations of CH₃NCH₃, CH₃NC groups and the

in table 1.

In normal Raman spectrum (figure 4A), the

wagging vibrations of CH₃, CH₃NCH₃groups.

The similar patterns are observed in SERS

highest peak emerges at 1456 cm^-1and five other spectra represented for Thiram-Ag₂₀(figure 4C) and

highly pronounced peaks are at 371, 576, 1012, 1414 Thiram-Au₂₀(figure 4D). Nevertheless, they

and 1430 cm^-1. They are responsible for the experience a slight westward-shift and register a

stretching vibrations of C–S, C=S, S–C–S and C–N lower overall intensity.

bonds accompanied with the scissoring bending

By interacting with the metallic clusters, certain

vibrations of CH₃and CH₃NC groups. Regarding the characteristic peaks of Thiram are enhanced. These

SERS spectra of Thiram-Cu₂₀(figure 4B), Thiram- especially include those at 553 cm^-1(which is nearly

Ag₂₀(figure 4C) and Thiram-Au₂₀(figure 4D), some negligible in normal Raman spectrum of Thiram),

Vietnam Journal of Chemistry

SERS chemical enhancement by copper - nanostructures:…

1012 cm^-1, 1414 cm^-1and 1531 cm^-1(table 1). The 1.2 to 2.4 times higher than those of Thiram-Ag₂₀

Raman intensities of these peaks see a respective and from 3.3 to 4.2 times higher than those of

rise of 122, 102, 50 and 175 times in the SERS Thiram-Au₂₀.

spectrum of Thiram-Cu₂₀complex. Otherwise, the

indices for Raman enhancement vary from 21 to 104

times for Thiram-Ag₂₀complex and from 12 to 54

times for Thiram-Au₂₀complex. The enhancement is

mainly due to the stretching vibrations of CH₃N, CN

groups and the wagging vibrations of CH₃,

CH₃NCH₃groups. However, some other peaks only

witness a marginal-to-non enhanced intensity, such

as those at 371, 576, 1430 and 1456 cm^-1(figure

4A). In particular, two peaks at 371 and 1430 cm^-1

are both disappeared in the SERS spectra obtained

from all three metal complexes. These peaks relate

to the stretching vibration of C–S bond and the

scissoring bending vibrations of CH₃, CH₃NC

groups (table 1).

In addition, the Raman intensity of highest peak

in SERS spectrum of Thiram-Cu₂₀at 982 cm^-1is 2

times higher than that of Thiram-Ag₂₀and 6 times

higher than that of Thiram-Au₂₀. The other

noticeable peaks of Thiram-Cu₂₀complex (i.e. 534,

1398 and 1526 cm^-1) also have higher Raman

Figure 4: (A) Raman spectrum of Thiram and SERS

activities than the ones of other complexes. Overall,

Raman figures obtained for Thiram-Cu₂₀are from

spectra of the most stable complexes: (B) Thiram-

Cu₂₀, (C) Thiram-Ag₂₀and (D) Thiram-Au₂₀

Table 1: Vibrational assignments of normal Raman spectrum of Thiram and SERS spectra of Thiram

adsorbed on Cu₂₀, Ag₂₀and Au₂₀clusters

Raman

SERS-Cu₂₀

279 (80.2)

–

445 (76.6)

490 (171.3)

534 (439.9)

571 (109.9)

874 (238.8)

SERS-Ag₂₀

236 (47.2)

–

SERS-Au₂₀

210 (60.0)

–

401 (20.1)

–

538 (122.3)

557 (66.3)

870 (38.4)

983 (339.7)

1090 (8.3)

1155 (26.5)

1267 (12.9)

1399 (248)

–

Assignments

ρ(CH₃), σ(NCS), σ(CSS)

υ(CS), σ(CH₃NC)

301 (6.0)

371 (14.3)

451 (2.5)

–

553 (3.6)

576 (24.9)

873 (3.3)

396 (39.9)

525 (129.1)

540 (356.1)

559 (206.1)

871 (105.2)

991 (984.2)

1092 (12.4)

1159 (23.3)

1274 (58.8)

σ(CS), σ(NCS), σ(CH₃NC),

ω(SCS), ω(CH₃NCH₃)

υ(SS), ω(SCS), ω(CH₃NCH₃)

σ(CH₃NCH₃), υ_s(SCS), υ_as(CSS)

υ_s(CH₃NCH₃), υ_s(CS)

υ_as(SCS), ω(CH₃), υ(CH₃N), σ(CH₃NC)

ρ(CH₃), ω(CH₃)

ω(CH₃), ρ(CH₃), υ_as(SC=S)

υ_as(CH₃NCH₃), ω(CH₃), υ_as(SCS)

υ(CN), ω(CH₃)

1012 (18.9) 982 (1924.5)

1102 (4.3)

1178 (1.8)

1297 (1.9)

1126 (103.9)

1161 (84.0)

1272 (112.6)

1414 (20.7) 1398 (1041.7) 1398 (440.6)

1430 (14.4)

1456 (50.4) 1455(157.6)

1531 (3.6) 1526 (630.4)

–

σ(CH₃)

υ(CN), ω(CH₃), σ(CH₃)

1457 (53.7)

1549 (375.8)

1458 (46.8)

1554 (193.3)

Values in parentheses are calculated Raman activities; (υ) = stretching (with υ_s= symmetric stretching and

υ_as= anti-symmetric stretching), σ = scissoring bending, ρ = rocking, ω = wagging, τ = twisting.

is based on the amplification of the light by the

excitation of localized surface plasmonic resonances

3.4. Chemical enhancement mechanism

It has been widely accepted that the SERS (LSPRs). The latter primarily refers to charge

phenomenon generally stems from electromagnetic transfer (CT) process, where the excitation

and chemical enhancement mechanisms. The former

wavelength resonates with the metalmolecule

Vietnam Journal of Chemistry

Dao Duy Quang et al.

charge transfer electronic states.^[17]The chemical

enhancement mechanism of Thiram adsorbed on

Au₂₀, Ag₂₀and Cu₂₀clusters is illustrated in figure 5

and table 2.

and LUMO one, i.e. HOMO-LUMO gap or E, is a

good indicator to evaluate kinetic stability and

chemical stability. Regarding table 2, the E values

of the bare clusters accord with the order: Au₂₀

In particular, figure 5 summarizes total density

of states (DOS) spectrum of Thiram, Au₂₀, Ag₂₀and

Cu₂₀bare clusters in comparison with the ones of

their complexes. The highest occupied molecular

orbital (HOMO) and the lowest unoccupied

molecular orbital (LUMO) distributions of the

studied structures are also displayed with their

corresponding energy values (E_Hand E_L). The

extended values of HOMO-LUMO energy gap (E)

are also included. Furthermore, partial density of

states (PDOS) analyses provides contributing

proportion of Thiram and its coordinated metal

clusters in the complexes. LUMO and HOMO are

also indicated in order to analyze the CT tendency of

electron densities.

Ag₂₀> Cu₂₀with their corresponding figures 1.89,

1.67 and 1.46 eV, respectively. The narrowest

energy gap of Cu₂₀(1.46 eV) indicates its highest

reactivity towards Thiram in comparison with Au₂₀

and Ag₂₀clusters. Expectedly, the HOMO-LUMO

energy gaps of the complexes are also in the similar

order: Thiram-Au₂₀(1.41 eV) > Thiram-Ag₂₀(1.28

eV) > Thiram-Cu₂₀(0.95 eV) which shows a reverse

order of the stability.

Thus, the narrower energy gaps of the Cu₂₀bare

clusters and of the Thiram-Cu₂₀complex are more

conducive transfer of electron densities from the

ligand to the cluster than those carried out by Ag₂₀

and Au₂₀clusters. The easier electronic transfer also

explains for the most marked enhancement by SERS

for Thiram adsorbed on Cu₂₀(figure 4 and table 1).

Firstly, the difference between HOMO energy

Figure 5: Density of states (DOS) spectrum of Thiram, Au₂₀, Ag₂₀and Cu₂₀bare clusters and their

complexes with Thiram (Thir-Au₂₀, Thir-Ag₂₀, Thir-Cu₂₀). LUMO and HOMO distributions are presented on

the right and left hand sides of each graphic. The LUMO and HOMO energies are indicated besides the

vertical dotted lines with their HOMO-LUMO gap (E) in eV unit. The percentage values correspond to 

contribution of Thiram and the metal clusters to LUMO and HOMO orbitals

Vietnam Journal of Chemistry

SERS chemical enhancement by copper - nanostructures:…

Secondly, information on the frontier orbitals of 4. CONCLUSIONS

the formed complexes clarifies the interaction

mechanism between the pesticide molecule and the Structural, electronic and spectroscopic properties of

clusters. And the most important interactions are Thiram and its complexes with Cu₂₀, Ag₂₀and Au₂₀

between HOMO and LUMO of Thiram and those of are computationally investigated using DFT method.

the clusters. Based on the energy gap, the donation or Normal Raman spectra of Thiram and SERS

back-donation of electrons can be revealed. Based on spectrum of its three complexes are projected. The

the calculated data in table 2, the energy differences results show that:

between LUMO of Thiram and HOMO of the clusters

are also ranged in declined order: Au₂₀> Ag₂₀> Cu₂₀

with the respective values being 4.31, 3.27 and 3.16

eV. In contrast, the energy gaps between LUMO of

the Au₂₀, Ag₂₀and Cu₂₀clusters and HOMO of

Thiram register are similar order but with

considerably smaller values i.e. 0.77, 1.59 and 1.49

eV, respectively. These imply that the HOMO-

LUMO energy gaps of the forward donation

(ThiramM₂₀, with M represents the metal cluster)

are larger than the ones represented for backward

donations (M₂₀Thiram). Hence, Thiram is adsorbed

on the metal cluster by donating its electron densities

to the cluster. This electron-transfer tendency from

organic molecules to metallic cluster is in agreement

with the previous studies.^[18-20]

+ Thiram contains two co-planar C₂NCS₂groups

and the interactive sites of Thiram are mainly found

at the its sulfur atoms (especially at S(sp²) atoms, i.e.

S3 and S4).

+ Thiram interacts with Cu₂₀cluster via two or

more sulfur atoms. The stability of Thiram-Cu₂₀

complexes depends on the number of interaction

between Cu₂₀cluster and S(sp²) atom. The more

S(sp²) atom interact with the Cu₂₀cluster, the more

stable the complex is.

+ Normal Raman spectrum of Thiram shows

several main peaks including the stretching vibration

of C–S bond and scissoring bending vibration of

CH₃groups. Otherwise, the main peaks of SERS

spectrum of Thiram-Cu₂₀, Thiram-Ag₂₀and Thiram-

Au₂₀complexes relate to N atom and the wagging

vibration of CH₃groups.

+ The SERS chemical enhancement for Thiram

derived by Cu₂₀cluster is 2 and 6 times higher than

those attained by Ag₂₀and Au₂₀clusters.

+ The most enhanced SERS signals of Thiram

adsorbed on Cu₂₀cluster are firstly related to its

lowest HOMO-LUMO energy gap by referencing to

the Au₂₀and Ag₂₀clusters. Moreover, during the

adsorption, the charge transfer prevails through the

forward donation direction from Thiram to the metal

clusters (ThiramM₂₀). The energy gap between

LUMO of Thiram and HOMO of Cu₂₀is the lowest

compared with those of Au₂₀and Ag₂₀cluster. The

highest charge transfer from Thiram to cluster is also

obtained for the copper one. And this tends to the

highest SERS signals obtained when Thiram is

adsorbed on the Cu₂₀cluster.

Table 2: HOMO and LUMO energies and HOMO-

LUMO energy gap

HOMO

-4.72

-5.84

-4.80

-4.69

-4.94

-4.26

-3.94

LUMO

-1.53

-3.95

-3.13

-3.23

-3.53

-2.98

-2.99

E (eV)

3.19

Thir

Au₂₀

1.89

Ag₂₀

1.67

Cu₂₀

1.46

Thir-Au₂₀

Thir-Ag₂₀

Thir-Cu₂₀

1.41

1.28

0.95

This observation is further confirmed by

analyzing partial density of states (PDOS) (figure 5).

The contribution percentages of Thiram and the

clusters to LUMO and HOMO indicate that electron

densities are always transferred from Thiram to the

cluster during the transition from LUMO to HOMO

of the complexes. Regarding Thiram-Cu₂₀, 75  of

LUMO electron density is localized on Thiram while

only 25  is found on the Cu₂₀cluster. However,

only 9  of HOMO electron density localizes on

Thiram, the corresponding figure for the cluster Cu₂₀

is 91 %. This means that 64  electron densities are

transferred from Thiram to the Cu₂₀cluster.

Consistent phenomena are observed in regard to

Au₂₀and Ag₂₀clusters with the transfer of 16  and

73  electron densities, respectively.

The predicted results suggest a magnification-

enhanced

and

cost-effective

copper-based

nanomaterial as a potential alternative for expensive

inert metals, such as silver or gold, in SERS

applications. The most noticeable downside is its

sensitivity to ambient oxidization. The disadvantage

is less pronounced if the material is expected for

portable or one-use purposes.

Acknowledgments. This research is funded by

Vietnam National Foundation for Science and

Technology Development (NAFOSTED) under grant

number 103.03-2018.366.

Vietnam Journal of Chemistry

Dao Duy Quang et al.

in North America, CRC Press, Boca Raton: CRC

Press, 2017.

Conflict of interest. The authors declare no conflict

of interest.

8. J. S. Kang, S. Y. Hwang, C. J. Lee, M. S. Lee. SERS

of dithiocarbamate pesticides adsorbed on silver

surface; Thiram, Bull. Korean Chem. Soc., 2002, 23,

1604-1610.

9. A. K. Verma, R. K. Soni. Silver nanodendrites for

ultralow detection of thiram based on surface-

enhanced Raman spectroscopy, Nanotechnology,

2019, 30, 385502-385516.

REFERENCES

1. R. Jayaraj, P. Megha, P. Sreedev. Organochlorine

pesticides, their toxic effects on living organisms and

their fate in the environment, Interdiscip. Toxicol.,

2016, 9, 90-100.

2. R. C. Gupta. Carbamate Pesticides, in: Encycl.

10. K. Rajalakshmi, M. Vetrivel. Optimization and

Toxicol., Elsevier, 2014, 661-664.

vibrational

study

2-propylpyridine-4-

3. V. K. Sharma, J. S. Aulakh, A. K. Malik, Thiram:

degradation, applications and analytical methods, J.

Environ. Monit., 2003, 5, 717-723.

carbothioamide by DFT - A theoretical study, Int. J.

ChemTech Res., 2016, 9, 306-316.

11. N. T. T. An, D. Q. Dao, P. C. Nam, B. T. Huy, H. N.

Tran. Surface enhanced Raman scattering of

melamine on silver substrate: An experimental and

DFT study, Spectrochim. Acta Part A Mol. Biomol.

Spectrosc., 2016, 169, 230-237.

4. B. Gupta, M. Rani, R. Kumar. Degradation of thiram

in water, soil and plants: A study by high-

performance liquid chromatography, Biomed.

Chromatogr., 2012, 26, 69-75.

5. H. Dies, M. Siampani, C. Escobedo, A. Docoslis.

Direct detection of toxic contaminants in minimally

processed food products using dendritic surface-

enhanced Raman scattering substrates, Sensors, 2018,

18, 1-11.

12. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.

Scuseria, M. A. Robb et al., Gaussian 16 Rev. A.03,

2016.

13. J. P. Perdew, K. Burke, M. Ernzerhof. Errata:

Generalized gradient approximation made simple,

Phys. Rev. Lett., 1997, 78, 1396.

14. M. K. Kesharwani, B. Brauer, J. M. L. Martin.

Frequency and zero-point vibrational energy scale

factors for double-hybrid density functionals (and

other selected methods): Can anharmonic force fields

be avoided?, J. Phys. Chem. A., 2015, 119, 1701-

1714.

6. P. Stathi, K. C. Christoforidis, A. Tsipis, D. G. Hela,

Y. Deligiannakis. Effects of dissolved carboxylates

and carbonates on the adsorption properties of

thiuram disulfate pesticides, Environ. Sci. Technol.,

2006, 40, 221-227.

7. G. M. Linz, M. L. Avery, R. A. Dolbeer, eds.

Ecology and Management of Blackbirds (Icteridae)

Corresponding author: Dao Duy Quang

Institute of Research and Development, Duy Tan University

3, Quang Trung, Da Nang, 50000, Viet Nam

E-mail: daoduyquang@duytan.edu.vn.

8 trang yennguyen 18/04/2022 61680

Download

Bạn đang xem tài liệu "SERS chemical enhancement by copper - Nanostructures: Theoretical study of Thiram pesticide adsorbed on Cu₂₀ cluster", để tải tài liệu gốc về máy hãy click vào nút Download ở trên

File đính kèm:

sers_chemical_enhancement_by_copper_nanostructures_theoretic.pdf